Cellular damage to aerobic organ tissues is well recognized as a consequence of ischemia, whether endogenous as in the case of a spontaneous coronary artery occlusion, or iatrogenic such as with open heart, coronary bypass surgery, or transplant procedures with the heart or other organs such as the lung, liver, kidney, pancreas and gastrointestinal tract. The degree and duration of the ischemia causing events are relevant to the amount of cell death and/or reversible cellular dysfunction. It is also known that much of the tissue damage in fact occurs upon reperfusion (i.e., resumption of blood flow) and re-oxygenation of the previously anoxic tissue. Reperfusion injury has been the subject of considerable recent study prompted by medical advances particularly in the treatment of reperfusion injury after myocardial infarction and other myocardial remedial procedures such as coronary bypass, other open heart surgeries, as well as organ transplants.
As a side product of normal aerobic respiration, electrons are routinely lost from the mitochondrial electron transport chain. Such electrons can react with molecular oxygen to generate the reactive free radical superoxide which through other reaction steps in the presence of hydrogen peroxide and iron produces the extraordinarily reactive and toxic hydroxyl radical. Metabolically active aerobic tissues possess defense mechanisms dedicated to degrading toxic free radicals before these reactive oxygen species can interact with cellular organelles, enzymes, or DNA, the consequences of which could, without such protective mechanisms, be cell death. These defense mechanisms include the enzymes superoxide dismutase (SOD) which disproportionates superoxide, catalase which degrades hydrogen peroxide, and the peptide glutathione which is a non-specific free radical scavenger.
While not fully understood, it is believed that with ischemia of metabolic tissues and subsequent reperfusion, a complex group of events occurs. Initially during the ischemic period, intracellular anti-oxidant enzyme activity appears to decrease, including that of SOD, catalase, and glutathione. There is also an indication that the level of xanthine oxidase activity concomitantly increases in vascular endothelial tissue during the ischemic event. The combination of enhanced ability to produce oxygen free radicals (via enhanced xanthine oxidase activity) and reduced ability to scavenge the same oxygen radicals (via reduced SOD, catalase and glutathione activity) greatly sensitizes the ischemic cell to an oxidative burst, and hence damage, should these cells be subsequently reperfused with blood and therefore oxygen. This oxidative burst occurring within seconds to minutes of reperfusion could result in reversible and irreversible damage to endothelial cells and other cells constituting the ischemic-reperfused organ matrix. If, for example, the heart is the organ under consideration, reversible oxidative damage can contribute to myocardial stunning, whereas irreversible damage presents itself as a myocardial infarction. Attendant with this initial oxidative burst is oxidation damage to cell membranes. Lipid oxidation in cell membranes appears to play a role in neutrophil chemotaxis to post-ischemic areas. Such activated neutrophils adhere to vascular endothelium, induce the conversion of xanthine dehydrogenase to xanthine oxidase within said endothelial cells, and further aggravate loss of endothelial integrity. Activated neutrophils also migrate out of the vasculature into myocardial interstitial spaces where the inflammatory cells can directly kill myocytes. Additionally, perturbations in normal calcium mobilization from sarcoplasmic reticulum as a consequence of ischemia-reperfusion contribute to reversible myocardial dysfunction referred to as myocardial stunning.
The consequences of ischemia-reperfusion events are reversible and irreversible cell damage, cell death, and decreased organ functional efficiency. More specifically, in the case of myocardial reperfusion injury, the consequences include myocardial stunning, arrhythmias, and infarction, and as a result, cariogenic shock and potentially congestive heart failure.
The paradox of cellular damage associated with a limited period of ischemic anoxia followed by reperfusion is that cell damage and death appear not only likely to directly result from the period of oxygen deprivation but, additionally, as a consequence of re-oxygenation of tissues rendered highly sensitive to oxidative damage during the ischemic period. Reperfusion damage begins with the initial oxidative burst immediately upon reflow and continues to worsen over a number of hours as inflammatory processes develop in the same post-ischemic tissues. Efforts dedicated to decreasing sensitivity of post-anoxic cells to oxidative damage and, additionally, efforts to reduce inflammatory responses in these same tissues have been shown to reduce the reversible and irreversible damage to post-anoxic reperfused organs. A combination of methods to reduce both the initial oxidative burst and subsequent inflammation associated damage could provide synergistic protection against reperfusion injury.
With respect to the treatment of ischemia coincident with MI patients, common therapies now used are to employ thrombolytics such as streptokinase and t-PA and angioplasty. U.S. Pat. No. 4,976,959 discloses the administration of t-PA and SOD to inhibit tissue damage during reperfusion and/or percutaneous transluminal coronary angioplasty coincident with ischemia to restore regional blood flow. Thus, an increasing number of patients are being exposed to the likelihood of reperfusion injury and its effects, particularly cardiac patients.
Reperfusion injury to organs other than the heart will generally manifest itself in substantially reduced efficiency of function, a consequence of which may be premature degeneration of the organ, or simply shutdown. Additionally, transplanted organs experience enhanced rejection rates if there is significant underlying reperfusion injury.
As discussed briefly above, while the precise mechanism of reperfusion injury has not been clearly defined, mounting data, most of which has been gathered in various cardiac model studies, indicate that the generation of oxygen-derived free radicals, including superoxide anion (O.sub.2).sup.-, the hydroxyl free radical (.OH) and H.sub.2 O.sub.2, results as a consequence of the reintroduction of molecular oxygen with reperfusion and plays an important role in tissue necrosis. Agents which either decrease the production of these oxygen derived free radicals (including allopurinol and deferroxamine) or increase the degradation of these materials such as superoxide dismutase, catalase, glutathione, and copper complexes, appear to limit infarct size and also may enhance recovery of left ventricular function from cardiac stunning.
The use of metabolic intervention as a therapy specifically during acute myocardial infarction is well established, although not without controversy. There is abundant experimental and clinical evidence to support the use of glucose-insulin-potassium (GIK) infusion--the primary form of metabolic intervention--after acute MI, particularly following the success of the Swedish DIGAMI study (Malmberg, K, and DIGAMI Study Group (1997) Prospective randomized study of intensive insulin treatment on long term survival after acute myocardial infarction in patients with diabetes mellitus. Brit. Med. J. 314, 1512-1515). The DIGAMI study emphasized the efficacy of a glucose-insulin infusion for acute MI in diabetic patients, but this type of therapy has never been suggested or used for reperfusion.
It therefore can be seen that there is a need for a safe effective composition having broad applicability to prevent or ameliorate the harmful effects of ischemia and reperfusion for tissues in general, especially organ tissue and, including but not limited to myocardium. It is primary object of the present invention to fulfill this need.
Another object of the present invention is to provide a method for treating ischemia and reperfusion without the side effects normally attendant with therapies presently available.
Still another object of the present invention is to provide a pharmaceutically acceptable carrier composition which can be used for intravenous administration of the compositions of the present invention without any significant undesirable side effects and without adversely affecting antigenic or immune stimulating properties.